I didn't have anything to add because we're still hoping the school will teach our child, but am interested in your replies too. One thing I've thought about with regard to their putting isolated work at the back of the classroom at the bottom of the list of options is combining it with tutoring at home. So I would choose a combination of both, so he gets group work in the cryptography, and mentored/self-paced work with the AoPS.
I don't think slowing down is a bad thing, and if you can do it with interesting challenging enrichment work, it sounds good to me.
